Free base jerking is the action of jerking off in unusual locations of your home when no one is home to judge you. A typical arrangement is that you put porn on in multiple rooms and run from room to room jerking off.

With this said no one really free base jerks. It is just a expression that shows the freedom in masturbating one has when no one is home.

I hear your roomates are gone for the week I bet you are free base jerking off everywhere.